1. Field of the Invention
The present invention relates to a frequency modulation circuit, and is applicable to, for example, the frequency modulation of a sound signal in an RF (radio frequency) modulator used for VTR (video tape recorder), CATV (community antenna television), and the like.
2. Description of the Related Art
Hitherto, in a VTR, for example, a video signal and a sound signal reproduced from a magnetic tape is converted in an RF conversion circuit into a high-frequency signal (RF signal), and supplied to an antenna terminal of a television. A VTR reproduces the video and sound signals converted to this high-frequency signal using the television.
The RF conversion circuit obtains an FM (frequency modulation) sound signal in which a sound carrier signal is frequency-modulated with a reproduced sound signal to a balanced-modulated RF signal with the FM sound signal, and simultaneously obtains an AM (amplitude modulation) video signal in which a RF signal is amplitude-modulated with a reproduced video signal to mix the AM video signal with a balanced modulation output. Then, a RF conversion circuit sends out the mixed signal through a band pass filter having a predetermined frequency as an RF conversion signal.
One of the RF conversion circuits of the aforementioned kind, as an FM circuit for obtaining the FM sound signal, has been disclosed in Japanese Patent Laid-Open No. 60-248008 (Japanese Patent Publication No. 4-79163). The circuit constitution of the FM circuit for realizing this is shown in FIG. 1. The FM circuit 1 outputs a sound carrier signal S1 oscillated from the LC resonance circuit 3 of a main oscillation circuit 2 as a signal S4 via a transistor Q5 and a coupling capacitor C2, and obtains a multiplication output by multiplying the signal S4 and the sound carrier signal S1 and an audio signal SA input from a sound signal input circuit 5 at a multiplication circuit 4. Then, the FM circuit 1 shifts the phase of the sound multiplication output via a phase shifting circuit 6, and then synthesizes it with the sound carrier signal S1, thereby obtaining the frequency modulation output. Then, FM circuit 1 outputs frequency-modulated signal from an output amplifier circuit 7.
The main oscillation circuit 2 connects a collector of a differential transistor Q7 out of two differential transistors Q6 and Q7 via an amplification transistor Q11 and an IC terminal T3 to the LC resonance circuit 3 composed of a coil L1 and a capacitor C1. The LC resonance circuit 3 is connected to an IC terminal T2 extending from a power source line P1, thereby sends out the sound carrier signal S1 from the LC resonance circuit 3 connected in parallel with a load resistor R6 to the side of the transistor Q11. The sound carrier signal S1 is fed through the feedback transistor Q5 and further through a coupling capacitor C2 back to the base of the other transistor Q6 by positive feedback, so that the transistors Q6 and Q7 oscillate at the oscillation frequency of the LC resonance circuit 3.
On the other hand, in the sound signal input circuit 5, the audio signal SA is input from an external terminal T4 to bases of transistors Q29 and Q30 of a differential amplifier, and is output from the collectors as sound signals S2 and S3 to the multiplication circuit 4. In the multiplication circuit 4, the sound carrier signal S4 sent from the transistors Q6 and Q7 of the main oscillation circuit 2 is amplitude-modulated by the sound signals S2 and S3, whereby an AM sound signal S5 is obtained. The AM modulation signal S5 is shifted .pi./2 radians by a capacitor C3, and then vector-synthesized in the emitter of the amplifier transistor Q11 with the sound carrier signal S1, so that a frequency modulation signal can be obtained.
The frequency of the sound carrier signal S1 oscillated from the LC resonance circuit 3 of the FM circuit 1 is fixed by the inductance of the coil L1, and the electrostatic capacity and the circuit impedance of the capacitor C1. For example, in the NTSC (national television system committee) method, it is set to oscillate a modulation signal of 4.5 MHz as a modulation frequency.
Incidentally, in addition to the NTSC method, various TV methods such as
(phase alternation by line) and the like are adopted in various regions of the world, so the sound carrier signal used in the FM circuit is set to a different frequency for each of TV systems. In Japan and the U.S., for example, the NTSC method is adopted and therefore a modulation frequency of 4.5 MHz is set. On the other hand, in European countries including U.K. and Eastern Europe, 5.5 MHz, 6.0 MHz or 6.5 MHz are employed as a modulation frequency, respectively. Therefore, in the FM circuits in which the resonance frequency of the LC resonance circuit is fixed such as the FM circuit 1 described above, there has been no interchangeability between regions in which the frequencies of the sound carrier signals are different.
Then, in order to correspond to each of modulation frequencies of these different TV methods, there has been proposed a FM circuit as shown in FIG. 2 in which a varactor diode D1 capable of varying an electrostatic capacity by voltage is combined with a coil L2 of an LC resonance circuit 8, and which makes a resonance frequency variable thereby performing modulation conversion.
However, there is a fundamental problem that the C-V (electrostatic capacity-voltage) characteristic of the varactor diode is a non linearly proportional relation. Also, there is another problem. If the amplitude of an input signal becomes large, a self-bias effect occurs which adds an oscillation signal voltage to the varactor diode, and therefore the linearity of the frequency modulation is deteriorated. Further, there is another problem that, in the cases where the bias to be supplied to the LC resonance circuit is set to 5 V or more to vary the electrostatic capacity of the varactor diode, an additional power supply is needed under the condition that the circuit such as IC is operated by 5 V, and therefore the circuit constitution becomes complicated.